factory_method.rb

class Wand
  def cast_spell
    spell = get_new_spell
    spell.cast! 
  end

  def get_new_spell
    raise "abstract method #get_new_spell must be defined"
  end
end

class HarryPottersWand < Wand
  def get_new_spell
    WingardiumLeviosa.new
  end
end

class SeverusSnapesWand < Wand
  def get_new_spell
    Crucio.new
  end
end